drm/amd/display: Fix Display Flicker on embedded panels

[Why]
ASSR is dependent on Signed PSP Verstage to enable Content
Protection for eDP panels. Unsigned PSP verstage is used
during development phase causing ASSR to FAIL.
As a result, link training is performed with
DP_PANEL_MODE_DEFAULT instead of DP_PANEL_MODE_EDP for
eDP panels that causes display flicker on some panels.

[How]
- Do not change panel mode, if ASSR is disabled
- Just report and continue to perform eDP link training
with right settings further.

Signed-off-by: Praful Swarnakar <Praful.Swarnakar@amd.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org